The present invention relates generally to a recording sheet, and more particularly to an inkjet recording sheet, the ink receptive coating thereon and the paper substrate supporting the ink receptive coating.
Paper for recording sheets used in inkjet printing must rapidly absorb the ink to reduce drying time with little or no backside show-through. Further, diffusion of ink laterally on the surface of the recording sheet must be prevented in order to achieve high resolution without blurring. Thus, for obtaining color images having good color density and resolution, with good absorbtivity and water fastness as well as optical brightness, a coated paper for inkjet printing should be able to achieve these results without any substantial dimensional change. For this purpose, the base paper for ink receptive coatings is generally made from bleached chemical pulp to which fillers, dyes, and if required, sizing agents and strength enhancers are added. An example of a typical paper basestock for use in the manufacture of an inkjet recording sheet is disclosed in U.S. Pat. No. 5,589,259.
It is also known that paper substrates for inkjet recording can be improved by applying an under coat or base coat to the paper surface before applying the ink receptive coating. Such base coats generally comprise a pigment and binder where the coated surface has a porous structure resulting in a large amount of pores or voids in the base coat layer. An example of a base coat for an inkjet recording sheet that is subsequently provided with an ink receptive layer is disclosed in U.S. Pat. No. 5,670,242.
Finally, the ink receptive coatings for inkjet paper must provide a surface that is receptive to the inks used for printing. In the past, this result has been achieved through the use of high pigment-to-binder ratios, usually in combination with pigments or coating materials that provide a porus and permeable coating layer. Because of the highly specific requirements for inkjet printing, coating materials used in other printing processes generally are unsatisfactory for inkjet printing.
There are two primary requirements for inkjet printing. The first is that the ink receptive coating, the substrate, and any intermediate base coat must be absorbent enough to immobilize the vehicle of the inks away from the surface so that the inks will not smear. The second requirement is that the ink receptive coating must provide a means for keeping the dyes in the ink on the surface with no spreading, tailing or blurring of the ink drops.
These requirements are achieved in the present invention with a combination of base coat and top coat where the components are matched to achieve a cooperative relationship not found in the prior art.
In conventional inkjet recording sheets, a generally porous fine powder capable of absorbing an ink is coated on a paper surface with a binder. However, when ink is brought into contact with such a coating, the coating is unable to instantaneously absorb the entire amount, and thus it takes a finite time for the inks to be absorbed. This allows the ink drop to spread in a fairly wide range among the particles of the fine powder. The color density tends to be low towards the forward edge of the spread, and the ink tends to spread unnecessarily widely, whereby the entire color density tends to be correspondingly low. This tends to make the sharpness of the printed image low and color blending or blotting is likely to result. However it has now been found possible to overcome these drawbacks by suppressing the unnecessary spread of the ink drop, and to provide a means for the ink drop including its vehicle to be absorbed into the paper substrate as a whole.
For this purpose, according to the present invention, porous particles are provided in both the ink receptive layer and in the base coat layer in a manner such that they work in a cooperative fashion to provide a superior image with good drying capacity.